The number of 15- to 17-year-old boys having sex in the past decade dropped 8.5 percent, and teens were generally acting more responsibly when it came to sex with rates of pregnancy, abortions and sexually transmitted diseases all falling.

These findings come from a new study by sociologists Barbara Risman of North Carolina State University and Pepper Schwarz of the University of Washington appearing in the premier issue of Context, a new journal published by the American Sociological Association.

The study analyzed data from the Center for Disease Control's Youth Risk Behavior Survey of more than 10,000 high school students nationally, as well as data from a number of other sources.

Analysis of the CDC data by Risman and Schwartz showed that the number of teens ages 15 to 17 who reported they had engaged in sexual intercourse dropped from 54.1 percent in 1991 to 48.4 percent 1997. Other data showed the teen pregnancy rate fell 14 percent to its lowest point since l975, and the abortion rate for teen-agers decreased 31 percent between 1986 and 1996.

Reasons for the drop in teen-age sex and related behaviors defy easy explanations, but "we are not getting lower rates of sex because high school kids are more conservative," said Schwartz. "And we are not going back to the values of the 1950s as some social commentators have claimed."

The reasons are more complex, but became clearer when Schwartz and Risman broke down the CDC data by gender. They found that while the overall rate for sexual activity among teens dropped 5.7 percent, most of the decrease was attributable to fewer boys having sex when they were 15 to 17 years old. The number of sexually active girls fell more modestly during the same 1991-1997 period, from 50.8 percent to 47.7 percent.

What did change in the 1990s was the context for high school sex. The authors write, "our best informed guess is that the cultural norms for girls' sexuality have dramatically changed. Girls are now presumed sexually active inside, but not outside, romantic relationships. Boys are therefore now much more likely to begin their sexual lives with a girlfriend."

This contrasts with pre-sexual revolution times when boys had their initial sexual encounter with so-called "bad" girls or "sluts" who were outside their social circle.

"Girls are not having less sex today, but they are insisting that they have it in a relationship," said Schwartz. "However, the definition of a relationship has changed. This drives parents crazy because a boyfriend can be like the flavor of the month rather than going with a guy for two years.

"A girl today might be called a 'slut' because she had sex outside a relationship, not because of the number of partners she had. Boys have to conform to this social context. A boy can't play around and have a girlfriend. His girlfriend may not be ready for sex so the boy delays his sexual activity or leaves the relationship."

The authors also speculate that girls' increasing influence over the conditions of sexual intercourse has contributed to an insistence on safer sex and increased use of more effective methods of birth control.

"Teen-agers are getting the message about safety," Schwartz said. "We are delighted that there are fewer teen-age moms and dads and that the rates of abortions and STDs are dropping."

However, she said most Americans are still in massive denial when it comes to conceding there is teen sexuality and being able to talk about it.

"I think we are grossed out at the idea of our kids being sexual, particularly adolescent sexuality," Schwartz said. "We have a visceral reaction to our daughters, in particular, being sexual beings the way we are. Fathers don't want their daughters treated the way they treated girls in high school. With boys there is less ambivalence. Parents are more worried about a son being trapped in an early marriage.

"The big issue for parents is acknowledging that during high school the majority of kids will have had intercourse and that most kids will be having sex by the time they hit 20.

"Parents don't want to deal with sexuality in their children. There is a lot of evidence that they will talk to their kids to protect them at 10 or 12, but by age 15 the media and friends become children's main source of information about sex. However, if you opt out of accepting your kids' sexual lives because of squeamishness you are depriving them of the understanding, support and access to a knowledgeable person they need. Parents need to learn how to deal with information that may be hurtful to them so they can have dialogue with their children."

Risman and Schwartz also examined the CDC data by race. Overall rates of sexually active high school students fell for whites, Hispanics and blacks between 1991 and 1997. For whites the drop was from 50.1 percent to 43.7 percent, for blacks it was from 81.5 percent to 72.7 percent and for Hispanics is was from 53.1 percent to 52.2 percent.

Rates for boys in all three groups declined, but the story among girls was more complex. Rates for white and Hispanic girls remained relatively stable although the rate of Hispanic girls increased slightly by 3.6 percent. Black girls, however, showed a 7.9 percent decrease. The data showed that among whites boys were less likely than girls to be sexually active by age 17. Black and Hispanic boys were more likely than girls to be sexual active, although the gaps are narrowing.